1. Field of the Invention
The present invention relates to dual-energy x-ray absorptiometry; and, in particular to an instrument for high-precision, calibrated, absorption measurements that support computations of bone structure, strength and risk of fracture.
2. Description of the Related Art
The system of bones and skeletal muscles provides structure to a human or animal body, and provides the capability to carry out activities. Bone provides the basic structural integrity of the body that carries forces and furnishes a framework for muscle.
Experience with bed rest subjects, astronauts and cosmonauts indicates that the magnitudes and patterns of bone tissue loss are extremely variable from one individual to the next, and also between different body regions. Little mass appears to be lost from the upper extremities during weightlessness; whereas the rate of mass loss from the vertebrae, pelvis, and proximal femurs of astronauts average between 1 percent and 1.6 percent per month. The rate of mass loss from those sites in postmenopausal woman average between 0.8 percent and 1.3 percent per year—a substantially lower rate of loss.
Recent evidence shows that there are important differences between the ways that bone is lost in aging on earth compared to changes observed during space flight. On earth, the skeleton is continually loaded during normal activities. Load causes mechanical strains within the bone, which tend to be greatest on the subperiosteal surface, the connective tissue with bone forming cells attached to the surface (cortex) of the bone. In response, more new bone mass forms on the cortex. Simultaneously, the normal turnover of bone accompanying the aging process causes some net loss of bone from endocortical (inside the cortex) and internal surfaces. In long bones, the net loss under loading causes skeletal strains to increase most on the subperiosteal surface, not at the internal surfaces where the bone loss occurred. Because it takes less new bone on the subperiosteal surface to compensate for bone loss from internal surfaces, strength can be maintained in the presence of net bone loss.
During space flight, loading is practically absent on the lower skeleton. Not only does bone loss accelerate under diminishing loading, but evidence from cosmonaut data suggests that the compensatory changes are absent as well. This means that astronauts may be at a greater risk of fracture for the same loss of bone mass. Therefore it is important not only to determine bone mass, but also to determine the geometrical configuration of the bone structure. Bones loss countermeasures can be developed to increase the loading on the lower skeleton. The efficacy of such countermeasures is better determined individually, based on the geometrical configuration of the individual's bone structure before and after the countermeasures, than by analyzing bone breakage statistics over a large population of astronauts. There is simply not a large population of astronauts.
Furthermore, the determination of bone structure is useful for screening a population and monitoring treatments of osteoporosis in postmenopausal women, elderly men and other susceptible individuals.
Loading and bone loss countermeasures can also be assessed through the measurements of muscle mass in a living human. Therefore it an advantage for a scanning device to also distinguish fat from muscle in soft tissue. Soft tissue excludes bone tissue.
There are several methods for determining bone mineral density (BMD), bone structure, and soft tissue components. These methods include computed tomography (CT), magnetic resonance imaging (MRI), ultrasound, and dual-energy x-ray absorptiometry (DXA).
While a CT unit can image and measure the geometrical characteristics of bone and soft tissue, it is not well suited for use in space because of its high radiation dose per scan. In addition, a CT unit capable of performing total body scans is extremely massive, weighing thousands of pounds. This great weight renders such units impractical for portable and space flight use. In addition, the high cost and large size place such units beyond the reach of small earthbound clinics, which might otherwise administer osteoporosis screening and treatment monitoring. An MRI unit is excellent for imaging soft tissues, for example to distinguish fat from muscle. However, an MRI unit suffers from a similar size and weight disadvantage. An MRI unit capable of performing whole body scans consumes significant power, generates large magnetic fields, and weighs tens of thousands of pounds.
Commercial scanners use dual-energy x-ray absorptiometry (DXA) or ultrasound to yield measurements of bone mineral density (BMD) that are regional averages. However, regional averages obscure structural details, and thus are not precise enough to deduce bone strength. Such systems do not predict risk of breakage. Furthermore, ultrasound devices have not been used successfully for the quantification of muscle mass.
In addition, commercial DXA devices consume too much energy for portable use. Furthermore DXA scanners employ ionizing radiation, which can pose a radiation risk to astronauts confined to operate in small spaces in the vicinity of a DXA device.
Based on the foregoing description, there is a clear need for a portable device that provides measurements of bone structure, fat tissue mass and lean tissue mass.
In particular, there is a need for a device that provides measurements of bone structure, fat tissue mass and lean tissue mass and that meets space flight constraints on size, weight, power consumption and radiation leakage.
Furthermore, there is a need for a system that yields a risk of injury including bone breakage.